It is well known that blood oxygen saturation can be measured optically. Devices for performing such measurement, known as oximeters, exist in the prior art; e.g., U.S. Pat. No. 3,799,672 to Vurek; U.S. Pat. No. 3,638,640 to Shaw.
The advantages of optical measurement of blood oxygen saturation levels during procedures, such as cardiopulmonary bypass during open heart surgery, are apparent. Utilization of extracorporeal blood circulation during such procedures facilitates optical monitoring of blood oxygen saturation and obviates the need for invasive withdrawal of samples for descrete measurements which aggravate blood loss and increase the risk of infection.
The prior art devices for optically measuring blood oxygen saturation levels by utilizing infra-red and red radiation, principally in the 6600 and 9000 Angstrom wavelength ranges, respectively, have constituted an advance in the art over single light source devices.
However, many of such devices cannot be adjusted to compensate for hematocrit fluctuations, with the result that readings are somewhat inconsistent among blood samples containing different hematocrit levels. Other devices can be manually adjusted to compensate for variations in hematocratic levels by varying the current to the light sources. This often results in non-uniformity of photon penetration depth into the blood sample, with less than optimum results. For instance, as a result of the inability of such a device to compensate for varying hematocrit levels, several things may occur. If the hematocrit level is low, the light tends to be transmitted through the blood, which is not optically dense, and reflected off the back wall of the cuvette from which it is transmitted via the blood to the light sensor, thus causing an erroneous reading. If the hematocrit level is comparatively high, the blood is optically dense and the light tends to reflect from the blood's surface, giving rise to error due to surface effects. In addition, the higher optical density of blood at higher hematocrit levels results in a lower amount of reflected light transmitted to the sensor, causing a relatively poor signal to noise ratio at the transducer level, which effect is multiplied by the signal processing and contributes significantly to error.
In addition, the accuracies of the prior art devices are limited by flow-direction, turbulence and microemboli effects, as well as by the efficiency of the optical coupling between the light sources, cuvette walls and photosensors.
Finally, some of the prior art devices are subject to error through the introduction of noise from various sources into the electronic circuitry which derives blood oxygen level readings from the reflected light energy.